1. Field of the Invention
This invention relates to the transport and storage of hazardous materials and, more specifically, to a segmented lid for a shielding cask adapted to receive radioactive or other hazardous material.
2. Background of the Invention
The nuclear and chemical industries produce large amounts of waste. The storage and transportation of these waste materials present an ever increasing challenge. Typically this waste material is stored at or near the point of production in standard size containers such as barrels that provide no shielding from radiation. Transportation for even very short distances requires encasing each container in a cask that will provide shielding.
While freshly produced reactor waste emits alpha-, beta-, and gamma-emissions, alpha emissions are very short lived. The beta rays emitted in the waste have very short path lengths but positrons emitted by the waste collide with surrounding electrons each collision producing two 0.511 MeV gamma rays as they annihilate in such a collision.
Therefore in storing or shipping radioactive waste one must shield for gamma rays, including specifically 0.511 MeV gamma rays.
The most effective shielding for gamma rays is provided by high atomic number materials (e.g. metallic lead) because of their high density. Lead also provides excellent shielding for beta rays. A lead shielded cask is suitable for the storage and transportation of a wide variety of hazardous materials.
When it is required to shield both gamma rays and neutrons, shielding comprising a mixture of lead pellets and metal hydride pellets (or another neutron absorber) is suitable. Also, metal hydride may be mixed with molten lead before lead pellets, shot, or brick are formed.
Typical shielding casks comprise a container vessel and a lid. The vessel has a stationary bottom and sidewalls containing an appropriate thickness of lead (typically 3 in.) and other shielding material. The cask must also comprise a movable lid with sufficient thickness of lead. Such a lid is very heavy. An edge of some lids is in hingeable communication with the container. This results in the lid having a large moment of inertia around that hinge with the lid swinging through a large volume of space above and alongside the vessel. At the 180 degrees point of the lid swing, the center of gravity of the combination (vessel+lid) is far removed from the center of gravity defined when the lid is in the closed configuration. This displacement of center of gravity threatens the equilibrium of the combination so as to cause tipping of the container.